Dissolution of sulfate scales

ABSTRACT

The invention provides for the use of compositions to dissolve sulfate scales from surfaces containing these scales, comprising an aqueous solution of an aminopolycarboxylic acid (APCA) containing 1 to 4 amino groups, e.g., diethylenetriaminepentaacetic acid (DTPA) or ethylenediaminetetraacetic acid (EDTA), or a salt thereof, and a second component which is diethylenetriaminepenta (methylenephosphonic acid) (DTPMP) or a salt thereof, or aminotri(methylenephosphonic acid) (ATMP), or a salt thereof. More particularly, the invention encompasses a method of removing sulfate scales from surfaces on which they have formed utilizing the foregoing solutions, particularly the removal of scales from equipment and wellbore surfaces associated with the operation of crude oil and gas wells.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method of dissolving sulfate scales fromsurfaces such as the equipment and wellbore surfaces of oil and gaswells.

2. Background Information and Description of Related Art

Sulfate scales normally form as a result of the mixing of incompatiblewaters. This may occur during crude oil and gas production when seawater containing sulfate and bicarbonate ions is mixed with reservoirwater containing alkaline earth metal cations (barium, calcium andstrontium). Depending on surrounding conditions such as pH, temperature,pressure and the presence of a seed crystal, if the concentrations ofalkaline earth cations and sulfate ions exceed the solubility of thecorresponding alkaline earth sulfates, the sulfate will precipitate andform scales. Under these conditions, the scales may also form with asingle source of water containing supersaturated levels of alkalineearth metal and sulfate ions.

Adherent and occluded scales cause serious problems in many applicationsinvolving water flow. For example, during gas and oil production, scaleplugging of surface and subsurface equipment, tubings, and perforationsoften lead to severe productivity decline and difficult operatingconditions. Of the sulfate scales, barium sulfate is particularlytroublesome because of its extreme insolubility. Furthermore, bariumsulfate is often associated with strontium sulfate which may carrytraces of radium with its potentially dangerous property ofradioactivity.

Different methods have been proposed for the removal of sulfate scalesincluding mechanical and chemical treatments. The latter treatmentsinclude contacting the scale with any of various agents in which thescale will dissolve, such as solutions of an aminopolycarboxylic acid(APCA), e.g. diethylenetriaminepentaacetic acid (DTPA) orethylendiaminetetraacetic acid (EDTA), which acts as a sequestrant.However, while the APCA's are somewhat effective in dissolving sulfatescales, including barium sulfate, further increases in both the rate ofdissolution and the amount of scale dissolved are considered highlydesirable from the standpoint of increased productivity. Thus an methodyielding such improvement, particularly with regard to barium andstrontium sulfate scales, could be of considerable importance.

Various improvements in the dissolution of sulfate scales have beenproposed, some of which are disclosed in the following prior artreferences which are cited and described herein in accordance with theterms of 37 CFR 1.56, 1.96 and 1.98.

U.S. Pat. No. 3,660,287, issued May 2, 1972 to Quattrini, discloses theremoval of scale including calcium and barium sulfates from oil wellequipment by treating it with a composition comprising a partiallyneutralized aminopolyacetic acid, e.g., DTPA or EDTA, and a carbonatesuch as ammonium bicarbonate.

U.S. Pat. No. 4,708,805, issued Nov. 24, 1987 to D`Muhala, discloses theremoval of barium and strontium sulfate scales by treatment with asequestering composition comprising citric acid, a polycarbazic acid,and an alkylenepolyaminopolycarboxylic acid, e.g., DTPA or EDTA.

U.S. Pat. No. 4,621,694, issued Nov. 11, 1986 to Wilson et al.,discloses a method for removing scale from wellbore surfaces andequipment utilizing a composition comprising an external aqueous phasein which is emulsified a hydrocarbon membrane phase enveloping aninternal aqueous phase, with the hydrocarbon and internal aqueous phasescontaining different complexing agents.

J. C. Cowan and D. J. Weintritt, Water-Formed Scale Deposits, (Houston:Gulf Publishing Co., 1976) 467 and 468, disclose thataminomethylphosphonates (AMP) can be used for treating scale and thatproducts based on AMP derivatives have been used to remove calciumsulfate scale in oilfield applications (page 467, col. 2). However,these authors also state that "AMP's and other organophosphorusderivatives cannot remove barium sulfate scale" (page 468, col. 2).

K. 0. Meyers et al., Journal of Petroleum Technology, June 1985,1019-1035 disclose the use of ATMP and DTPMP (referred to in the articleas DETPMP) as threshold scaleinhibitors in preventing or minimizing theformation of calcium carbonate scales. A threshold scale inhibitor is acompound capable of preventing scale formation at concentrations farbelow that required to sequester the scale compound which is necessaryfor dissolution.

SUMMARY OF THE INVENTION

In accordance with this invention, new compositions are provided for thedissolution of sulfate scales from surfaces containing these scales,such compositions comprising an aqueous solution of anaminopolycarboxylic acid (APCA) containing 1 to 4 amino groups, e.g.,diethylenetriaminepentaacetic acid (DTPA) or ethylenediaminetetraaceticacid (EDTA), or a salt thereof, and a second component which isdiethylenetriaminepenta(methylenephosphonic acid) (DTPMP) or a saltthereof, or aminotri(methylenephosphonic acid) (ATMP), or a saltthereof. The invention also encompasses a method of removing sulfatescales from surfaces on which they have formed utilizing the foregoingsolutions. While the inventive compositions can be utilized for thedissolution of sulfate scales from any source, they are particularlyuseful in the removal of such scales which form on equipment andwellbore surfaces in the production of crude oil and gas due to themixing of incompatible waters.

DESCRIPTION OF THE DRAWING

The drawing shows curves which indicate the effect on barium sulfatedissolution of solutions of DTPA alone, and combinations of DTPA andDTPMP or ATMP.

DESCRIPTION OF PREFERRED EMBODIMENTS

The solution used to dissolve the sulfate scale may containconcentrations of an APCA or salt thereof and DTPMP 0R ATMP or saltthereof, of, for example, about 0.1 to 1.0 M, preferably about 0.3 to0.6M of each material. The APCA and PBTC may be added in the form of thefree acids or as salts, preferably the potassium salts. However, ifadded as the free acid, it is desirable to add a basic material, e.g.,potassium or cesium hydroxide or carbonate, preferably potassiumhydroxide, to at least partially change the free carboxyl groups toanionic carboxylate, i.e., salt groups. Whether the APCA and DTPMP orATMP are added as salts or as free acids which are reacted with a basicmaterial in situ, it is preferred that the solution applied to the scalehave a basic pH, e.g. about 8 to 14, more preferably about 11 to 13.

The sulfate scale intended to be dissolved under this invention containsa substantial proportion, e.g., at least about 15 wt. % of alkalineearth metal sulfates, i.e., sulfates of calcium, strontium, barium andin many cases a trace amount of radium. Other scale forming compoundssuch as calcium carbonate and other alkaline earth carbonates are oftenalso present. The inventive method is particularly useful for thedissolution of sulfate scales wherein the alkaline earth metal sulfatescomprise a significant proportion, e.g., at least about 50 wt. %., ofbarium sulfate which is particularly insoluble and difficult to removeby hitherto known methods.

While an aqueous solution of APCA and DTPMP or ATMP can be used as is todissolve the sulfate scale in many situations, it may be advantageous insome instances to employ such solutions as part of a liquid membranesystem, as described in previously cited U.S. Pat. No. 4,621,694, theentire disclosure of which is incorporated by reference. As previouslydescribed, this type of system involves a composition comprising anexternal aqueous phase in which is emulsified a hydrocarbon membranephase enveloping an internal aqueous phase. The external aqueous phasemay contain a relatively weak chelating or complexing agent such as thesodium salt of EDTA; the hydrocarbon membrane phase contains acomplexing agent stronger than any which may be present in the externalaqueous phase, e.g., a di-long chain alkylnaphthalene sulfonic acid suchas dinonylnaphthalene sulfonic acid, didecylnaphthalene sulfonic acid,or didodecylnaphthalene sulfonic acid; and the internal aqueous phasecontains a complexing agent stronger than in the hydrocarbon membranephase, e.g., a pentasodium salt of DTPA.

As an embodiment of the invention claimed herein, the described liquidmembrane system contains as the internal aqueous phase a solution of anAPCA and DTPMP or ATMP, or salts thereof, as hereinbefore disclosed. Inpractice, the components of the sulfate scale contacted by the emulsiontend to dissolve in the external aqueous phase, such dissolution aidedby any relatively weak complexing agent which is present. The dissolvedalkaline earth metal ions then tend to migrate to the hydrocarbonmembrane phase because of the greater complexing power of the agentcontained in such membrane phase. Finally, the ions tend to migrate fromthe membrane phase to the internal aqueous phase because of the stillgreater complexing power of the agent in the latter phase. This type ofmembrane system can have greater dissolving power than a straightsolution of the relatively strong complexing agent utilized in theinternal aqueous phase because the described migration of the alkalineearth metal ions prevents the concentration of such ions from buildingup too quickly in the external aqueous phase thus allowing for theirdissolution over a longer period of time. Moreover, the use in theinternal aqueous phase of the combination of APCA and DTPMP or ATMPrather than other agents such as the APCA alone provides for moreefficient scale removal by the system because of the greater dissolvingand complexing power of such combination. Note that the choice of APCAin the combination of agents utilized in the internal aqueous phase maydepend on the strength of the agents employed in the external aqueousand hydrocarbon membrane phases. Thus if an EDTA salt is employed as theagent in the external aqueous phase as illustrated in U.S. Pat. No.4,621,694 previously cited, then the APCA used in the combination ofagents employed in the internal aqueous phase will generally be strongerthan the EDTA salt. e.g., a DTPA salt.

In dissolving sulfate scale from surfaces under this invention, thescale may be contacted with the composition comprising APCA and DTPMP orATMP using any means known in the art. For example, in the case of anoil or gas well, the composition may be injected into the wellbore andthrough the tubular equipment, e.g., piping, casing, etc., andpassageways, which have been fouled by the scale, optionally after beingpreheated, e.g., to a temperature between about 25° C. to about 100° C.,although the temperatures prevailing downhole may make preheatingunnecessary. Once within the tubular goods and the passageways requiringtreatment, the composition may be allowed to remain there for a periodof time to dissolve all or a significant proportion of the scale, e.g.,about 10 minutes to about 7 hours. After being in contact with theequipment for the desired time, the solution containing the dissolvedscale is withdrawn and may be discarded or re-injected into thesubsurface formation as is or after treatment to remove dissolved ionsand/or adding more APCA and DTPMP or ATMP. This procedure can berepeated as often as required to remove scale from the equipment.

In one procedure for circulating the composition through the tubulargoods in the well, the composition is pumped down through the productiontubes and returned to the surface through the annular space between theproduction tubes and the casing (or vice versa). Also, the compositionmay be pumped down through the production tubing and into the formation,thereby cleaning the well, including the well casing, and the formationpore space by dissolving sulfate scale present as it flows over andalong the surfaces that need cleaning. The spent composition containingthe dissolved, complexed barium together with any other alkaline earthmetal cations which may have been present in the scale, includingradium, may then be returned to the surface, for example, bydisplacement or entrainment with the fluids that are produced throughthe well after the cleaning operation. In an alternative manner, thecomposition may be applied batchwise fashion, for example, by injectingthe composition into the well and optionally into the pore spaces of theadjacent earth formation and there keeping the composition in contact innon-flowing condition with the surfaces that are covered with scale, fora period of time sufficient to dissolve the scale.

In another embodiment of the inventive method of scale removal,removable production equipment containing such scale may be placed in avessel and completely immersed in the composition until the scale isdissolved.

The present scale removal technique is very effective for loweringresidual radioactivity of equipment contaminated with radium-containingbarium sulfate scale. Thus, radium may be precipitated in scale withother alkaline earth metals with the result that scaled equipment isoften radioactive to the point that it cannot safely be used. Using thepresent scale removal compositions, activity can be reduced toacceptable levels in comparatively short times without furthertreatment. Some residual activity arises from lead and otherradio-isotopes which are not dissolved in the composition; theseisotopes are decay products of radium and have originally beenincorporated in the scale with the radium and other alkaline earth metalsulfates. Although they are not removed chemically by the present scaleremoval techniques, the dissolution of the barium scale together withthe other alkaline earth metal sulfates enables these other componentsof the scale to be removed by simple abrasion, for example, by scrubbingwith or without a detergent/water scrub solution. In this way, theresidual activity level may be reduced to a very low value, below theappropriate regulatory standards. Thus, by using the present chemicalscale removal technique in combination with a simple mechanical removalof loose, non-adherent material, previously radioactive pipe may quicklyand readily be restored to useful, safe condition.

Although the invention has been described primarily with respect to theremoval of sulfate scales from the equipment and surfaces connected withoil and gas wells, it can also be applied to other surfaces prone to theformation of such scales, e.g., the interior surfaces of boilers, heatexchangers, condensers, flow lines, treaters, cooling towers, internalcombustion engines, and other equipment utilized in operationsnecessitating the handling of large amounts of water such as pulp andpaper mills and certain types of mining.

In order to demonstrate the barium sulfate scale-dissolving capacitiesof the composition, several aqueous solutions have been examined insimulated tests, the results of which are shown in the followingexamples.

Comparative Example A

This example shows the barium sulfate scale dissolving capacity of 0.5MDTPA used alone as a sequestrant.

The pH of an aqueous solution of 0.5M DTPA was adjusted to 12.0 by theaddition of potassium hydroxide. A sample of this solution (150 ml)containing 60.0 g/l of barium sulfate was refluxed for 7 hours undervigorous agitation. During the run, three samples of approximately 3 to4 grams in size were taken at 1, 3 and 7 hours. The samples werefiltered while hot through a 0.45 micron Acrodisc filter and thefiltered samples analyzed for barium by atomic absorption. At the end of7 hours, the remaining contents were filtered while hot through a 0.45micron filter. The solids were rinsed with 0.1M KOH and methanol anddried overnight at 100° C. under vacuum. The thus recovered solids wereweighed indicating that the 0.5M DTPA solution dissolved 47.2 g/l ofbarium sulfate in 7 hours at 100° C.

Example 1

This example, included within the claimed invention, shows theunexpected improvement in barium sulfate dissolving capacity when DTPMPis utilized with DTPA.

The procedure of Comparative Example A was followed except that anaqueous solution of 0.5M DTPA and 0.1M (or 1.0N) DTPMP was utilized. Itwas found that 49.6 g/l of barium sulfate was dissolved after 7 hours at100° C., an increase of 5.1% over the use of 0.5M DTPA alone, shown inComparative Example A.

Example 2

This example shows the unexpected improvement in barium sulfatedissolving capacity when ATMP is utilized with DTPA.

The procedure of Comparative Example A was utilized except that anaqueous solution of 0.5M DTPA and 0.167M (or 1.0N) ATMP was utilized. Itwas found that 48.8 g/l of barium sulfate dissolved after 7 hours, anincrease of 3.4% over the use of 0.5M DTPA alone.

Example 3

This example shows that the use of DTPA with ATMP in a concentrationhigher than 0.167M yields a further improvement in barium sulfatedissolution capacity.

The procedure of Example 1 was followed except that ATMP was used in aconcentration of 0.5M (or 3.0N) with the 0.5M DTPA. The amount of bariumsulfate dissolved after 7 hours at 100° C. was 53.6 g/l, an increase of13.6% over the use of 0.5M DTPA alone, as shown in Comparative ExampleA.

The results of Comparative Example A and Examples 1 to 3 are graphicallydepicted in the drawing in which the points on the curves indicate theamounts of barium sulfate dissolved in the various solutions of theexamples after 1, 3 and 7 hours of reflux at 100° C., determined byatomic absorption. The curves show that for the average period ofreflux, the amounts of barium sulfate dissolved in the solutions are inthe following order:

    0.5M DTPA<0.1M DTPMP/0.5M DTPA=0.167M ATMP/0.5M DTPA <0.5M ATMP/0.5M DTPA

The curves also show that with all the solutions, over 90% of thedissolution obtained after 7 hours of reflux, was achieved after thefirst hour.

Comparative Examples B and C

These examples illustrate that unlike the case with DTPMP or ATMP, twoother well-known polyphosphonic threshold scale inhibitors reduce ratherthan enhance the barium sulfate dissolving capacity of an APCA such asDTPA.

The procedure of Examples 1 to 3 was followed except that instead ofDTPMP or ATMP, the following commercial polyphosphonic threshold scaleinhibitors in 0.2N concentration were utilized with the 0.5M DTPA:ethylenediaminetetra (methylenephosphonic acid), sold under thetrademark "Dequest 2041" (Example B); and hexamethylenediaminetetra(methylenephosphonic acid) sold under the trademark "Dequest 2051"(Example C). The amounts of barium sulfate dissolved after refluxing at100° C. for 7 hours are shown in the following table.

    ______________________________________                                                         BaSO.sub.4 Dissolved,                                        Comparative Example                                                                            g/l                                                          ______________________________________                                        B                45.2                                                         C                37.6                                                         ______________________________________                                    

The results show that the use of the commercial threshold scaleinhibitors used with DTPA in Comparative Examples B and C reduce theamount of barium sulfate dissolved in 0.5M DTPA used alone (47.2 g/l),by 4.2% for "Dequest 2041" and 20.3% for "Dequest 2051". Thus the effectof DTPMP or ATMP in enhancing the barium sulfate dissolving capacity ofan APCA such as DTPA is very specific and unpredictable.

I claim:
 1. A composition for the removal of sulfate scale from surfacescomprising an aqueous solution of about 0.1 to 1.0 molar concentrationof an aminopolycarboxylic acid (APCA) containing 1 to 4 amino groups ora salt thereof, and about 0.1 to 1.0 molar concentration of a secondcomponent which is diethylenetriaminepenta(methylenephosphonic acid)(DTPMP) or a salt thereof, or aminotri(methylenephosphonic acid) (ATMP)or a salt thereof as an internal phase enveloped by a hydrocarbonmembrane phase which is itself emulsified in an external aqueous phase,said hydrocarbon membrane phase containing a complexing agent weaker forthe cations of said sulfate scale than said APCA and DTPMP or ATMP, anycomplexing agent for said cations in said external aqueous phase beingweaker than that in said hydrocarbon membrane phase.
 2. The compositionof claim 1 wherein said APCA is diethylenetriaminepentaacetic acid(DTPA).
 3. The composition of claim 1 wherein said APCA isethylendiaminetetraacetic acid (EDTA).
 4. The composition of claim 1wherein said second component is DTPMP or a salt thereof.
 5. Thecomposition of claim 1 wherein said second component is ATMP or a saltthereof.
 6. The composition of claim 2 wherein said second component isDTPMP or a salt thereof.
 7. The composition of claim 2 wherein saidsecond component is ATMP or a salt thereof.
 8. The composition of claim3 wherein said second component is DTPMP or a salt thereof.
 9. Thecomposition of claim 3 wherein said second component is ATMP or a saltthereof.
 10. A method of removing sulfate scales from surfaces on whichthey have formed comprising contacting said surfaces with an aqueoussolution of about 0.1 to 1.0 molar concentration of anaminopolycarboxylic acid (APCA) containing 1 to 4 amino groups or a saltthereof, and about 0.1 to 1.0 molar concentration of a second componentwhich is diethylenetriaminepenta(methylenephosphonic acid) (DTPMP) or asalt thereof, or aminotri(methylenephosphonic acid) (ATMP) or a saltthereof.
 11. The method of claim 10 wherein said concentration is about0.3 to 0.6M.
 12. The method of claim 10 wherein said solution has a pHof about 8 to
 14. 13. The method of claim 12 wherein said pH is about 11to
 13. 14. The method of claim 12 wherein said APCA and second componentare at least partially in the form of their potassium salts.
 15. Themethod of claim 4 wherein said aqueous solution is an internal phaseenveloped by a hydrocarbon membrane phase which is itself emulsified inan external aqueous phase, said hydrocarbon membrane phase containing acomplexing agent weaker for the cations of said sulfate scale than saidAPCA and second component, and any complexing agent for said cations insaid eternal aqueous phase being weaker than that in said hydrocarbonmembrane phase.
 16. The method of claim 10 wherein said sulfate scalecomprises at least about 15 wt. % of alkaline earth metal sulfates. 17.The method of claim 16 wherein said alkaline earth metal sulfatescomprise at least about 50 wt. % of barium sulfate.
 18. The method ofclaim 10 wherein said surfaces are equipment and wellbore surfacesassociated with the operation of a crude oil or gas well.
 19. The methodof claim 10 wherein said APCA is DPTA.
 20. The method of claim 10wherein said APCA is EDTA.
 21. The method of claim 10 wherein saidsecond component is DTPMP or a salt thereof.
 22. The method of claim 10wherein said second component is ATMP or a salt thereof.
 23. The methodof claim 19 wherein said second component is DTPMP or a salt thereof.24. The method of claim 19 wherein said second component is ATMP or asalt thereof.
 25. The method of claim 20 wherein said second componentis DTPMP or a salt thereof.
 26. The method of claim 20 wherein saidsecond component is ATMP or a salt thereof.
 27. The composition of claim1 wherein said APCA in said internal aqueous phase is DTPA and saidexternal aqueous phase contains EDTA as the sole complexing agent. 28.The composition of claim 1 wherein said complexing agent in saidhydrocarbon membrane phase is dinonylnaphthalene sulfonic acid,didecylnaphthalene sulfonic acid, or didodecylnaphthalene sulfonic acid.29. The method of claim 15 wherein said APCA in said internal aqueousphase is DTPA and said external aqueous phase contains EDTA as the solecomplexing agent.
 30. The method of claim 15 wherein said complexingagent in said hydrocarbon membrane phase is dinonylnaphthalene sulfonicacid, didecylnaphthalene sulfonic acid, or didodecylnaphthalene sulfonicacid.